The quality of the surface of various types of sheet material is often a very important parameter during production and subsequent processes which add value to the sheet material. As an example, paper products have certain characteristics, such as roughness, pore structure and linting which can significantly impact on production quality and down-stream processing (such as printing). By way of example, the background to this invention is further illustrated below by reference to the importance of roughness in paper products. However, this should not be construed so as to limit the scope of the present invention.
The ‘roughness’ (or conversely the ‘smoothness’) of paper is an important property that affects the papers's printability, appearance and texture. Roughness can manifest itself in various ways, such as in the surface profile, texture, pore structure or linting properties of the paper.
Roughness is affected by factors such as the furnish used to make the paper, surfaces with which the paper is contact in during manufacture, applied coating and the calendering process. ‘Calendering’ is a process used in papermaking to smooth the paper by passing it through one or more nips between two steel rolls, or through one or more nips between one steel and one polymeric or cotton covered roll.
Roughness is commonly measured using laboratory instruments based on the air-leak principle. Air-leak instruments measure roughness by placing a ring on top of the paper surface using a pre-selected force. Pressurised air escapes from the inside of the ring to the atmosphere through the gaps between the paper's surface and the ring. A rougher sheet has more or larger air gaps, and so the flow rate of air lost in this way is increased. The flow rate is measured and used as a basis for determining a roughness value. Three commonly used air-leak instruments include the Bendtsen, Sheffield and Parker Print Surf testers. Such air-leak instruments are commonly employed and relatively quick to use. However, they only provide limited roughness information, and the results are thought to be affected by air escaping through the pores in the sheet as well as through the sheet's surface roughness. The ring can also deform the surface of the paper as the measurement involves contact between the ring and the paper. In addition, air-leak devices do not describe the scale of the roughness, i.e. how the roughness of the sheet varies over the plane of the sheet, and whether this variability is periodic.
Paper roughness is less commonly measured using stylus-based instruments. A stylus is passed over the paper surface and its deviation is measured. Although this technique is capable of providing a three-dimensional description of the paper's roughness, it is rarely used due to it being relatively slow and not sufficiently robust for a paper mill's test room. There is also a possibility that pressure on the stylus will distort the roughness information due to the surface of the sheet being compressed against the stylus surface during measurement.
An alternative to the stylus instrument is to pass a triangulating laser over the paper's surface to determine the deviation of the surface. While the laser does not distort the surface, there is a question over whether the laser detects the paper's surface or a point beneath the paper's surface as light does not perfectly reflect from paper. As with stylus based-instruments, laser-based roughness measurement is slow and not routinely used for quality control purposes.
Paper roughness can also be measured by image analysis of cross-sectional images of paper sheets. This method is accurate, however only a very small area of the surface of the paper can be practically measured in this way. Moreover, specialist equipment and skills are required to perform the analysis.
An alternative approach for measuring paper roughness is described in U.S. Pat. No. 4,019,066. A beam of light is projected onto a moving paper web at a relatively low angle to obliquely illuminate the surface of the paper. Light reflected from the illuminated surface passes into a collector, whereupon it is converted into an electrical signal by an optoelectronic arrangement. The signal is then resolved into alternating and direct current components, and a roughness index is calculated by multiplying a scaling coefficient with the ratio of the alternating to the direct current component.
The importance of illuminating the surface at a relatively low angle is recognised in U.S. Pat. No. 4,019,066. Light striking the surface at a low angle serves to accentuate the contrast between the light and shadows cast by the “landscape” features on the surface. Shadows cast from light striking the surface at other than optimum angles tends to provide less information about the surface when the shadows are analysed. The approach taken in U.S. Pat. No. 4,019,066 to generate ‘useful’ shadows is to precisely arrange the light source at a particular angle of illumination and to only illuminate a very small area of the surface. The collector also includes an aperture to further limit collected light to that reflected from a surface area of around 1/100 of a square millimeter.
Precision alignment of the light source to the sheet-surface is cumbersome, and maintaining the angle of illumination can be difficult, particularly when the apparatus is employed in the paper mill environment. Moreover, like air-leak instruments, the apparatus of U.S. Pat. No. 4,019,066 can not readily analyse how the roughness varies across the plane of the sheet.
Therefore, it would be advantageous to provide an apparatus that overcomes these shortcomings of the prior art.
In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date:    (i) part of common general knowledge; or    (ii) known to be relevant to an attempt to solve any problem with which this specification is concerned.